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Extinction of Counterflow Diffusion Flames With Fine-Water Droplets.


pdf icon Extinction of Counterflow Diffusion Flames With Fine-Water Droplets. (781 K)
Lazzarini, A. K.; Krauss, R. H.; Chelliah, H. K.; Linteris, G. T.

Halon Options Technical Working Conference. Proceedings. HOTWC 2000. Sponsored by: University of New Mexico, Fire Suppression Systems Assoc., Fire and Safety Group, Great Lakes Chemical Corp., Halon Alternative Research Corp., Hughes Associates, Inc., Kidde Fenwal, Inc., Kidde International, Modular Protection, Inc., Next Generation Fire Suppression Technology Program, Sandia National Laboratories, Summit Environmental Corp., Inc. and 3M Specialty Materials. May 2-4, 2000, Albuquerque, NM, 195-203 pp, 2000.

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For more information contact: Center for Global Environmental Technologies, New Mexico Engineering Research Institute, University of New Mexico, 901 University Blvd., SE, Albuquerque, NM 87106-4339 USA.
Telephone: 505-272-7250,
Fax: 505-272-7203. WEB: http://nmeri.unm.edu/cget/confinfo.htm

Keywords:

halon alternatives; diffusion flames; extinction; droplets; water; drop sizes; size distribution; water vapor; halons

Abstract:

Basic investigations aimed at better understanding the fire suppression mechanism of water dates back to the 1950s, while more recent studies have focused on water mist systems. Although there is a consensus in the literature on the fundamental fire suppression mechanism of water, no detailed quanlitative information on the various physical. thermal, and chemical effects of water mist were available until recent modeling capabilities were developed. The recent detailed modeling efforts were primarily carried out in two configurations, counter-flow and co-flow. The former flow configuration provides a convenient approach to understanding the interactions of fine-water droplets with flames, including flame extinction conditions. For example, investigations by Lentati and Chelliah have shown that dilution of the air stream (or displacement of oxygen) with saturated water vapor alone reduces the flame extinction condition (characterized here by the flow strain rate) of a methane-air non-premixed flame by about 12%. Experimental results presented here support such predictions. The further addition of water, in the form of fine droplets, causes significant thermal cooling of the flame front because of the relatively large latent heat of vaporization of water. Addition of 3% of water by mass in the form of 20 mum monodisperse droplets (the optimum size for this flow configuration) was shown to reduce the extinction strain rate by an additional 55%. By selectively excluding the source terms contributing to the gas-phase and the condensed phase conservation equations, the importance of thermal effects associated with water mist was clearly demonstrated. The chemical (e.g., shifting of the water-gas equilibrium reaction and enhanced three-body recombination effects) and other physical (e.g., modification of transport coefficients) effects associated with fine-water droplets were shown to have a minor effect.